The present invention relates to the field of semiconductor devices and their manufacture. More specifically, in one embodiment the invention provides a bipolar transistor and a resistor on a single substrate and a process for their fabrication, using a single layer of polysilicon.
Previous methods have been available for producing transistors and resistors on a substrate. One advantageous method of producing such devices involves using a single layer of polysilicon which is selectively doped to perform the needed functions. The resulting devices can include only bipolar transistors and resistors or can include other types of devices such as CMOS devices to provide a "BiCMOS" product. BiCMOS devices offer the advantages of the high packing density and low power consumption of CMOS devices, as well as the high speed of bipolar devices. One BiCMOS device and process for fabrication thereof is described in U.S. Pat. No. 4,764,480 (Vora), assigned to the assignee of the present invention.
Previous applications of bipolar transistors and resistors using single layer polysilicon techniques have included selectively providing a layer of silicon nitride to selectively mask the resistor during subsequent processing. Previous fabrication techniques, however, have resulted in certain undesirable effects. A certain amount of the polysilicon's line width was lost during oxidation or dopant drive-in processes. Additionally, the transistor base resistance was relatively high.